As we discussed earlier the IO540 C4B5 has a different power rating than the IO-540 D4A5 recommenced by Van's Aircraft. These are 250 HP at 2575 rpm for the C4B5 and 260 HP at 2700 rpm for the D4A5. Everything I have read points to the fact that they are the same basic engine and capable of running at either rpm/power rating. It's just Piper derated the C4B5 by running it at a lower rpm for the Aztec. If you would like to explore more about the different Lycoming 540 series aircraft engine try this link Lycoming 540 reference.
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